Mosquito-transmitted alphaviruses, such as chikungunya virus (CHIKV) and Ross River virus (RRV), cause debilitating inflammation of joint and skeletal muscle tissue in people worldwide. Utilizing a mouse model of RRV-induced disease, studies have demonstrated critical roles for macrophages and the complement system in initiation of RRV-induced inflammation and promotion of the tissue destructive phase of the disease. Macrophages have broad proinflammatory, tissue destructive, and tissue remodeling/reparative capabilities, however, the host and viral mechanisms that regulate these effector functions following infection in vivo is not well understood. Our preliminary studies indicate that the complement system regulates the activation phenotype of RRV-induced inflammatory macrophages and disease partly through CR3, a signaling receptor that binds the complement activation product iC3b, suggesting that RRV-induced complement activation triggers a gene expression program within inflammatory macrophages that is associated with severe disease and tissue pathology. In other work, we have identified a unique RRV strain that replicates similar to the virulent strain in vitro and in vivo, yet fails to induce inflammation in joint and muscle tissue, suggesting that specific viral genetic elements contribute to the elicitation of a severe host inflammatory response independent of effects on in vivo replication. The specific aims of this application will use in vivo, genetic, and molecular approaches to i) investigate the role of CR3 in regulation of the activation phenotype of RRV-induced inflammatory monocytes/macrophages during distinct phases of alphavirus-induced inflammatory disease, ii) investigate whether C3aR and/or C5aR, additional receptors for complement activation fragments, promote RRV-induced disease, and iii) identify viral genetic elements required for RRV-induced complement activation and/or immunopathologic macrophage inflammation. Relevance: Arthritis/myositis-associated alphaviruses are an emerging threat due to their ability to initiate explosive epidemics and to cause disease in new areas. The studies proposed here will help understand how both the virus and the immune system contribute to severe inflammation that is detrimental to the infected person. Because inflammation is a central characteristic of many human diseases, both viral and nonviral, this work may provide general insight into how beneficial or harmful inflammation is regulated.